[AIRFLOW-3973] Commit after each alembic migration

[eeshugerman]

If Variables are used in DAGs, and Postgres is used for the internal
database, a fresh $ airflow initdb (or $ airflow resetdb) spams the
logs with error messages (but does not fail).

This commit corrects this by running each migration in a separate
transaction.

[ashb]

As mentioned in our other PR https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst#airflow-git-branches -- this PR needs to target master, not v1-10-test/stable please.

[eeshugerman]

This is already in master. I'm trying to backport to v1.10. Please see the discussion here: #4797

(Sorry for the confusion, I should have added a note of explanation to these.)

[ashb]

Ah, we don't need a PR in this case -- just marking the original PR for the milestone 1.10.11 is enough and we'd cherry pick it.

Since you've got it opened I'll merge this though.

(We like cherry-picks to have the "(cherry picked from commit ...)" footer that -x adds, and our tooling to work out which commits are yet to be backported for a release relies upon the (#<PR>) on the end of the commit, so I have manually done both of these for this PR.)

[eeshugerman]

👍 I see, thanks, I was wondering how that works.